Process for separating rare earths



Jan. 27, 1970 F. TROM BE .ETAL 3,492,084

PROCESS FOR SEPARATING RARE E'ARTHS Filed Aug. 29, 1966' 2 Shee'ts-Sheetl La o Ge 0 Pea 760 mmNg Pau 502 mm Hg Pee 250mm s Jam 7; 1970 FiledAug. '29, 1966 'IOON 75Nd Y 50 Nd SOY F. TROMBE ETAL 3,492,084

PROCESS FOR SEPARA'IING RARE mamas 2 Sheets-Sheet 2 g/L Sc 0; F lla 3 i760 mm Hg PCO 502 mm Hg I Pco ABOmn-Hg 010 2050 40 7o 9010ONd UnitedStates Patent Int. C1. 6221; 59/00 US. CI. 23-15 Claims ABSTRACT OF THEDISCLOSURE A mixture of rare earths is separated into a fractioncontaining rare earths whose carbonates are relatively more soluble anda fraction of rare earths whose carbonates are relatively less soluble,by contacting a mixture of rare earth carbonates and soluble rare earthsalts in the presence of carbon dioxide.

The invention pertains to a process for separating a mixture of rareearths (in which willbe included elements such as yttrium and scandium)into its individual components.

According to the process of the invention, one react a suspensionconsisting of a mixture of carbonates of the rare earth elements toseparate them from one another, in the presence of carbon dioxide, withan aqueous solution of soluble salts of the same rare earth elements.

This aqueous solution may have initially the same rare earth elementcomposition as said suspension.

It has been found, on the one hand, that the solubilities in an aqueoussolvent of most of the diiferent rare earth elements, though remainingvery low, increases together with the concentration of carbon dioxide inthe aqueous solvent and that, on the other hand, the differences in thesolubilities with respect to one another. of the respective rare earthelements increase as a function of the concentration of carbon dioxidedissolved in the aqueous medium. Broadly speaking, the solubilities ofthe respective rare earth elements considered increase with their atomicnumbers in the presence of dissolved carbon dioxide in the aqueoussolvent, the lightest, such as lanthanum and cerium being the mostinsoluble and the heaviest, such as erbium, being the most soluble. Thesolubility of the yttrium carbonate is intermediate between those of thecarbonates of the rare earths neodynium and erbium while scandiumcarbonate finds its place among the most soluble canbonates of theheaviest rare earths.

These phenomena are illustrated by the curves of FIG- URE 1 which showthe variations of the solubilities of the carbonates of the rare earthelements in water, at 22 C., in function of the pressure of carbondioxide in contact with the solution.

The measurements were made in a vessel, starting from pure rare earthcarbonates suspended in water (2 or 3 grams per litre), by subjectingthem to the action of carbon dioxide (in pure form or dilutmi withargon) for 4-5 hours at a predetermined temperature, in the presentinstance at 22 C., under increasing pressures of C0 The solutionrecovered in each experience was treated with HCl, heated to boiling tocompletely remove C0 The dissolved rare earth salt was then precipitatedin the form of the hydroxide with ammonia and calcined into oxides whichWere then Weighed.

FIGURE 1 shows clearly the increase in the solubility (plotted on theordinate and expressed in mg. per litre of the corresponding oxides) ofmost of the rare earth carbonates (as a function of the absolutepressure of canbon dioxide plotted on the abscissa in mm. of mercury)and further shows the increasing differences in the 'ice solubilitieswith respect to one another of the different carbonates as a function inthe increase of the pressure of carbon dioxide above the aqueoussolution.

It has been noted that, when the CO pressure is increased beyond oneatmosphere, the solubilities of the difierent carbonates remain quiteunchanged, except for the carbonates of the heaviest rare earth elementssuch as erbium (and scandium) the solubilities of which keep increasingslightly when the carbon dioxide pressure is increased up to 3atmospheres.

It has been further found that the solubilities of the rare earthcarbonates in the presence of a determined pressure of carbon dioxideincrease as the temperature decreases. The lower the temperature, thehigher the solubility of the rare earth carbonates.

FIGURES 2 and 3 show respectively the variations of the solubilities ofthe yttrium carbonate and scandium carbonate respectively as a functionof the temperature in the presence of carbon dioxide under pressures of250, 380, 502 and 760 mm. of mercury.

However, these solubilities remain in any case so low that theseparation of rare earths by a simple selective dissolution of a mixtureof their carbonates in water may not be carried out on an industrialscale as is evidenced by the experiences described herebelow which,nevertheless, show the important modifications of the composition ofsolubilized rare earths fraction as compared with the startingcomposition of the mixture.

The starting product consisted of a mixture of rare earth hydroxideshaving the composition shown in the middle column of Table I herebelow.Four batches, each of which consisted of five kilograms of hydroxides(containing about 70% of rare earth oxides), were treated with carbondioxide which was allowed to bubble into the suspension, this treatmentbeing repeated several times for removing soluble impurities and thecalcium. The bubbling of CO was continuously maintained under a pressureof 3 atmospheres in order to avoid hydrolysis back to the hydroxides ofthe carbonates obtained.

More specifically, these batches were suspended under vigorous stirringin l. of Water within a stainless steel vessel filled with carbondioxide under a pressure of 3 atmospheres. After a 24 hour contactbetween the suspension and the carbon dioxide, the solutions recoveredin the 4 experiences contained from 17 to 24 gr. (per 150 l. of water)of rare earth and yttrium oxides, e.g. from 117 to 166 mg./l.

The composition of the rare earth mixture extracted from the liquidphase is shown in the right hand side column of Table I (the rare earthswere dosed by X- fluorescence by means of the borax bead process).

TABLE I Composition after reaction Proportions expressed Proportionsexpressed in oxide weight perin oxide weight percent of the respectivecent of the respecrareearth elements tive rare earth in the carbonateelements in the Elements mixture solution Not dosed Not dosed It will benoted that the solution is highly enriched in yttrium. This table thusconfirms that there is a remarkable difference in solubilities between,for instance, the carbonate of neodynium and the carbonate of yttrium.

The yield of this enrichment operation is, however, extremely low and isof no practical use, due to the minor solubilities of the rare earthcarbonates.

However, according to the process of the invention, advantage is takenof these differences in solubilities for separating the rare earthelements from a mixture thereof by reacting a suspension of the mixtureof the rare earth elements in the form of their carbonates, in thepresence of carbon dioxide, with an aqueous solution of soluble salts ofthe same elements, this solution containing, if the case should be, thesame initial cationic proportions of said elements, whereby the solidphase suspended becomes more concentrated in those of the rare earthswhose carbonates exhibit the smallest solubilities and the solution inthe salts of the elements whose carbonates exhibit the greatestsolubilities.

It will be noted that the process according to the invention takesadvantage of the so-called metatectic reactions which are known formetallic hydroxides.

As a matter of fact, when a soluble salt XA of a cation X and aninsoluble salt YB of a cation Y, are brought into contact, the cations Xand Y, on the one hand, and the anions A and B, on the other hand, beingselected such that the salt XB is much more insoluble than the salt YB,there occurs the following reaction:

which causes cation Y to go into solution.

Such a type of reaction is known in connection with the hydroxides ofcopper, zinc and cadmium which exhibit, respectively, the followingsolubility products:

C-u(OH) lO Zn(OH) 10- Cd(OH) l The corresponding nitrates however arehighly soluble in water.

The reaction of zinc hydroxide or cadmium hydroxide suspended in waterwith the soluble copper nitrate causes the complete dissolution of thezinc or cadmium in the form of their nitrates according to the followingreaction:

In the practice of the process of the invention the raw materialconsisting of a mixture of hydroxides such as, for instance, the mixtureconsidered in the previous selective dissolution example (see middlecolumn of Table II), is first suspended in water and converted intocarbonates in the presence of carbon dioxide and reacted with anotherportion of the above starting mixture which has been previously treatedwith a dosed solution of the hydrochloric acid to convert the rare earthinto soluble chlorides.

Alternatively the process may also be carried out by first suspendingthe starting mixture of hydroxides in an aqueous medium and by addingthereto an amount of hydrochloric acid (or if the case should be ofanother acid the rare earth salts of which are soluble) dosed fordissolving only part of hydroxides mixture.

The suspension is then subjected to carbonation by bubbling carbonicdioxide through the suspension for a time sufficient to enable themetatectic reaction to take place.

It will be of course appreciated that the molar proportion ofhydrochloric acid used with respect to the quantity of rare earth oxidestreated will act on the final distribution conditions of the rare earthcations in the solid phase as well as in the solution.

In order to illustrate the foregoing, a non limitative example will begiven hereafter for illustrating the process according to the invention.

The starting mixture consisted of kg. of the hydroxides, mixture usedalready in the above disclosed examples and which, through calcination,yields 3.5 kg. of

Composition after metatectic raotiou Elements Composition in oxideweight percent of solid carbonate Composition expressed in oxide weightpercent of the mixture solution It will be noted that the proportions ofyttrium and, as a general rule, of the other rare earths with respect toone another in the solution is about the same than the one obtained inthe former example through simple selective dissolution of the startinghydroxides in the presence of carbon dioxide. However, the quantity ofrare earths extracted from the same volume of the solution obtained,starting from the same mixture, passes from 24 gr. to 510 gr., thus toan extracted quantity which has been multiplied by a factor of about 20,such multiplication enabling the application of this process toindustrial separations, the same steps being repeated either on theseparated carbonate mixture or on the mixture recovered fromthesolution, for instance by reaction with ammonia to transform againthe solubilized rare earth chlorides into the hydroxides.

By modifying the above example factors (such as a different proportionof HCl or a decrease of the temperature under which the process isoperated at a temperature close to 0 C.) this multiplication factor maybe highly increased up to a one hundred value. Other acids, such as HNOmay replace HCl.

This process is particularly eifective for separating for instance amixture of neodynium and yttrium. An example of this separation will bedescribed in connection with FIGURE 4 which, diagrammatically,represents the variations of the compositions of the liquid phase (curveI) and of the solid phase (curve II) in equilibrium therewith, as afunction of the composition of the starting mixture (whose varyingcompositions in yttrium and neodynium are represented by points on therepresented diagonal line of the square diagram) when the startingmixture of neodynium and yttrium hydroxides, suspended in water, hasbeen treated with the amount of hydrochloric acid which dissolves 20.8percent of the starting mixture and has further been subjected to thecarbon dioxide bubbling until completion of the reaction.

For instance, assuming that the starting mixture contains 30% ofneodynium and 70% of yttrium (the composition of this mixture beingrepresented by point a on the diagonal line of the square diagram), thefinal compositions of the liquid phase and solid phase will respectivelybe represented by points 0 and b (which compositions are read on theordinate) at the intersection of a line parallel to the ordinate passingby a with the two curves I and II.

It will be appreciated that the solid phase became richer in the elementwhose carbonate is the less soluble (it contains about 55% of neodyniumand 45% of yttrium) whereas the solution became richer in yttrium whosecarbonate is the more soluble (it contains about 20% of neodynium and ofyttrium).

The extraction operation may be repeated either on the solid phase(whose starting composition is represented by 0 on the diagonal line) oron the mixture of hydroxides precipitated, for instance with ammonia,from the liquid phase (whose starting compositions is represented by bon the diagonal line).

In the first instance a more highly neodynium enriched precipitateexhibiting the composition e is obtained in equilibrium with a solutionwhose composition is represented at d.

In the second instance a more highly enriched yttrium solutionexhibiting the composition 1 is obtained in equilibrium with a solidphase whose composition is represented at d.

It will be readily appreciated that, by repeating these operations a fewtimes on the precipitates obtained in the first instance and on thesolution obtained in the second instance, a pure solid neodymiumcarbonate phase and a pure yttrium chloride solution respectively willbe obtained rapidly.

In some instances, the separation may be perfected, in a final step, bypassing solutions of the soluble chlorides of the purified rare earthfraction through a column of ion exchange resin as is well known in theart.

There is thus provided a new process enabling an efficient separation atlow cost and by using the most conventional reaction materials.

What we claim is:

1. A process of separating a mixture of rare earths into a firstfraction enriched with respect to rare earths Whose carbonates are moresoluble and a second fraction enriched with respect to rare earths whosecarbonates are less soluble comprising the steps of contacting asuspension comprising a plurality of rare earth carbonates, in thepresence of carbon dioxide, with a solution comprising a plurality ofsoluble rare earth salts, at least some of the rare earths elements ofsaid soluble salts and of said carbonates being the same, to provide aliquid phase containing soluble rare earth salts enriched with respectto rare earths whose carbonates are more soluble and a solid phaseenriched in rare earths Whose carbonates are less soluble, said carbondioxide being present in an amount sufficient to permit a substantialseparation of said rare earths.

2. A process according to claim 1, wherein said solution contains saidrare earth soluble salts in the same proportions with respect to oneanother as in said mixture of carbonates.

3. A process according to claim 1, wherein said rare earth soluble saltsconsists of rare earth chlorides.

4. A process of separating a mixture of rare earths into a firstfraction enriched with respect to rare earths Whose carbonates are moresoluble and a second fraction enriched with respect to rare earths whosecarbonates are less soluble comprising the steps of providing asuspension of a plurality of rare earth hydroxides in water, treatingsaid suspension with an inorganic acid to convert a portion only of saidrare earth hydroxides into soluble rare earth acid salts, and contactingthe resulting reaction m'nrture with carbon dioxide to provide a solidphase enriched in respect to rare earths whose carbonates are lesssoluble and a liquid phase enriched in respect to rare earths whosecarbonates are more soluble, said carbon dioxide being present in anamount sufiicient to permit a substantial separation of said rareearths.

5. A process according to claim 4 wherein said inorganic acid compriseshydrochloric acid.

References Cited UNITED STATES PATENTS 2,872,287 2/1959 Dufi'ield et al.2322 X 3,092,449 6/1963 Bril et al. 23-23 X 3,153,571 10/1964 Bronaugh2361 X 3,401,008 10/1968 Head 23-23 HERBERT T. CARTER, Primary ExaminerUS. 01. X.R.

